[sfnt] Improve paint limit checks

Paint tables can appear before the `base_glyphs_v1` offset if the
font is produced with the layer list before the base glyph list.  In
this case paint tables can occur after the layer list but before the
base glyph list.  Checks in the 'COLR' v1 code were rejecting fonts
with this layout.  Improve these checks by calculating a minimum
offset after which paint tables can occur and use that in safety
checks.

* src/sfnt/ttcolr.c (Colr, tt_face_load_colr): Declare
`paint_start_v1` and calculate that as the minimum of the end of
layer list and base glyph list.
(get_child_table_pointer, read_paint, tt_face_get_paint_layers):
Use that in safety checks.

2 files changed, 43 insertions, 8 deletions

diff --git a/ChangeLog b/ChangeLog
index 8c079d101..131e7c9e2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,21 @@
+2021-06-28  Dominik Röttsches  <drott@chromium.org>
+
+	[sfnt] Improve paint limit checks
+
+	Paint tables can appear before the `base_glyphs_v1` offset if the
+	font is produced with the layer list before the base glyph list.  In
+	this case paint tables can occur after the layer list but before the
+	base glyph list.  Checks in the 'COLR' v1 code were rejecting fonts
+	with this layout.  Improve these checks by calculating a minimum
+	offset after which paint tables can occur and use that in safety
+	checks.
+
+	* src/sfnt/ttcolr.c (Colr, tt_face_load_colr): Declare
+	`paint_start_v1` and calculate that as the minimum of the end of
+	layer list and base glyph list.
+	(get_child_table_pointer, read_paint, tt_face_get_paint_layers):
+	Use that in safety checks.
+
 2021-06-28  Alexei Podtelezhnikov  <apodtele@gmail.com>
 
 	[raster] Clean up vertical sweep.
@@ -202,7 +220,7 @@
 
 2021-06-09  Alexei Podtelezhnikov  <apodtele@gmail.com>
 
-	* src/truetype/ttinterp.c (TT_RunIns): Optimize tracing. 
+	* src/truetype/ttinterp.c (TT_RunIns): Optimize tracing.
 
 2021-06-09  Alexei Podtelezhnikov  <apodtele@gmail.com>
 
diff --git a/src/sfnt/ttcolr.c b/src/sfnt/ttcolr.c
index 8db774c0d..1e297ac8c 100644
--- a/src/sfnt/ttcolr.c
+++ b/src/sfnt/ttcolr.c
@@ -41,7 +41,7 @@
 
   /* NOTE: These are the table sizes calculated through the specs. */
 #define BASE_GLYPH_SIZE                   6U
-#define BASE_GLYPH_V1_RECORD_SIZE         6U
+#define BASE_GLYPH_PAINT_RECORD_SIZE      6U
 #define LAYER_V1_LIST_PAINT_OFFSET_SIZE   4U
 #define LAYER_V1_LIST_NUM_LAYERS_SIZE     4U
 #define COLOR_STOP_SIZE                   6U
@@ -83,6 +83,13 @@
     FT_ULong  num_layers_v1;
     FT_Byte*  layers_v1;
 
+    /*
+     * Paint tables start at the minimum of the end of the LayerList and the
+     * end of the BaseGlyphList.  Record this location in a field here for
+     * safety checks when accessing paint tables.
+     */
+    FT_Byte*  paints_start_v1;
+
     /* The memory that backs up the `COLR' table. */
     void*     table;
     FT_ULong  table_size;
@@ -170,7 +177,7 @@
       p1                 = (FT_Byte*)( table + base_glyphs_offset_v1 );
       num_base_glyphs_v1 = FT_PEEK_ULONG( p1 );
 
-      if ( num_base_glyphs_v1 * BASE_GLYPH_V1_RECORD_SIZE >
+      if ( num_base_glyphs_v1 * BASE_GLYPH_PAINT_RECORD_SIZE >
              table_size - base_glyphs_offset_v1 )
         goto InvalidTable;
 
@@ -185,8 +192,18 @@
       p1            = (FT_Byte*)( table + layer_offset_v1 );
       num_layers_v1 = FT_PEEK_ULONG( p1 );
 
+      if ( num_layers_v1 * LAYER_V1_LIST_PAINT_OFFSET_SIZE >
+             table_size - layer_offset_v1 )
+        goto InvalidTable;
+
       colr->num_layers_v1 = num_layers_v1;
       colr->layers_v1     = p1;
+
+      colr->paints_start_v1 =
+        FT_MIN( colr->base_glyphs_v1 +
+                  colr->num_base_glyphs_v1 * BASE_GLYPH_PAINT_RECORD_SIZE,
+                colr->layers_v1 +
+                  colr->num_layers_v1 * LAYER_V1_LIST_PAINT_OFFSET_SIZE );
     }
 
     colr->base_glyphs = (FT_Byte*)( table + base_glyph_offset );
@@ -367,7 +384,7 @@
 
     child_table_p = (FT_Byte*)( paint_base + paint_offset );
 
-    if ( child_table_p < colr->base_glyphs_v1                          ||
+    if ( child_table_p < colr->paints_start_v1                         ||
          child_table_p >= ( (FT_Byte*)colr->table + colr->table_size ) )
       return 0;
 
@@ -388,7 +405,7 @@
     if ( !p || !colr || !colr->table )
       return 0;
 
-    if ( p < colr->base_glyphs_v1                          ||
+    if ( p < colr->paints_start_v1                         ||
          p >= ( (FT_Byte*)colr->table + colr->table_size ) )
       return 0;
 
@@ -608,7 +625,7 @@
        * skip `numBaseGlyphV1Records` by adding 4 to start binary search
        * in the array of `BaseGlyphV1Record`.
        */
-      FT_Byte  *p = base_glyph_begin + 4 + mid * BASE_GLYPH_V1_RECORD_SIZE;
+      FT_Byte  *p = base_glyph_begin + 4 + mid * BASE_GLYPH_PAINT_RECORD_SIZE;
 
       FT_UShort  gid = FT_NEXT_USHORT( p );
 
@@ -704,7 +721,7 @@
     /*
      * First ensure that p is within COLRv1.
      */
-    if ( p < colr->base_glyphs_v1                          ||
+    if ( p < colr->layers_v1                               ||
          p >= ( (FT_Byte*)colr->table + colr->table_size ) )
       return 0;
 
@@ -731,7 +748,7 @@
 
     p_paint = (FT_Byte*)( colr->layers_v1 + paint_offset );
 
-    if ( p_paint < colr->base_glyphs_v1                          ||
+    if ( p_paint < colr->paints_start_v1                         ||
          p_paint >= ( (FT_Byte*)colr->table + colr->table_size ) )
       return 0;